Evaluation of Network Data Aggregation

ABSTRACT

In one embodiment, a method includes receiving topology data that indicates multiple communication links and multiple intermediate network nodes in communication based on the communication links. The intermediate network nodes include multiple leaf nodes that terminate communications and multiple transit nodes that facilitate the passage of information between leaf nodes. Aggregation point data is also received, which indicates all aggregation points on the intermediate network nodes. An aggregation point is an interface between a network node and a communication link, through which is output data that is a combination of data received through multiple different interfaces upstream of the interface. A set of paths is determined for which each path in the set connects a different pair of leaf nodes. A measure of aggregation is determined based on a number of aggregated paths of the set of paths. An aggregated path passes through an aggregation point.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure relates generally to objective measures of designfor communication networks.

2. Background

Networks of general purpose computer systems and specialized devicesconnected by external communication links are well known and widely usedin commerce. The networks often include one or more network devices thatfacilitate the passage of information between the computer systems anddevices. A network node is a network device or computer or specializeddevice connected by the communication links. An end node is a networknode that is configured to originate or terminate communications overthe network. An intermediate network node facilitates the passage ofdata between end nodes.

Communications between nodes are typically effected by exchangingdiscrete packets of data. Information is exchanged within data packetsaccording to one or more of many well known, new or still developingprotocols. In this context, a protocol consists of a set of rulesdefining how the nodes interact with each other based on informationsent over the communication links. According to internetwork protocols,each node is given a logical internetwork address and intermediatenetwork nodes called routers track which internetwork address isreachable through which communication link. A well known internetworkprotocol is the Internet Protocol (IP). Information used by the routersis distributed using one or more of several well known routingprotocols. A well known routing protocol is Open Shortest Path First(OSPF) which exchanges full topology information about every node andcommunication link in an area.

To reduce the consumption of network resources and improve scalability,some routing protocols divide a large network up into smallersubnetworks. By aggregating routing information, the amount of networkresources consumed to maintain routing data and make routing decisionscan be reduced and network scalability can be enhanced. For example,OSPF divides a large network up into multiple areas and exchanges fulltopology information only within one area. At a boundary with adifferent area, address reachability data is aggregated and exchangedwith an adjacent node in the different area.

The connected communications links and division of routers into areas isa manual process performed by human network administrators. As a result,the division is subjective based on the administrator's perceptions andis not guaranteed to be optimal in any objective sense. As networksbecome larger, sub-optimal divisions can lead to significant wastedresources and increased costs to service the same customer base for agiven network. In some circumstances, sub-optimal divisions can lead toinstability and lack of resiliency in the network.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1A illustrates an example hierarchical network;

FIG. 1B illustrates an example full mesh network;

FIG. 1C illustrates an example preferred network;

FIG. 2A illustrates example data rate aggregation through an interfaceof a node;

FIG. 2B illustrates example topology aggregation through an interface ofa node;

FIG. 2C illustrates example address aggregation through an interface ofa node;

FIG. 3 illustrates an example method to evaluate and improve dataaggregation in a network; and.

FIG. 4 illustrates a computer system upon which an embodiment of theinvention may be implemented.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Techniques are described for evaluation of data aggregation in anetwork. In the following description, for the purposes of explanation,numerous specific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be apparent, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, well-knownstructures and devices are shown in block diagram form in order to avoidunnecessarily obscuring the present invention.

Applicants determined that an objective measure is desirable, which canbe used to compare aggregation associated with different designs forconnections among routers and division of routers into areas. Such ameasure can be used to certify network designers and to evaluate theevolution of a network over time. Applicants also determined that notonly is topology aggregated in network areas, but data rate and addressspace can also be aggregated at various points throughout a network andsuch aggregation should also be measured.

Embodiments of the invention are described in the contexts of routersthat perform data rate aggregation, topology aggregation or addressspace aggregation, or some combination, in a communications network.However, the invention is not limited to these contexts. In otherembodiments, other aggregation can be performed on data passed throughan interface of the same or different intermediate network nodes, suchas hubs and bridges. For example, in various embodiments, network costdata, such as noise, travel time, travel time variability (jitter), isaggregated, and non routing data, such as data packet payload data, isaggregated in a communications network.

1.0 Overview

In one set of embodiments, a method includes receiving topology datathat indicates multiple communication links and multiple intermediatenetwork nodes in communication based on the communication links. Thenodes include multiple leaf nodes that terminate communications andmultiple transit nodes that facilitate the passage of informationbetween leaf nodes. Aggregation point data is also received. Aggregationpoint data indicates all aggregation points on the intermediate networknodes. An aggregation point is an interface between a network node and acommunication link, through which is output data that is a combinationof data received through multiple different interfaces upstream of theinterface. A set of paths is determined for which each path in the setconnects a different pair of leaf nodes. A measure of aggregation isdetermined based on a number of aggregated paths of the set of paths. Anaggregated path passes through an aggregation point.

In other embodiments, an apparatus, or logic encoded in one or moretangible media, or instructions encoded on one or more computer-readablemedia is configured to perform one or more steps of the above method.

2.0 Network Overview

As stated above, communications between nodes are typically effected byexchanging discrete packets of data. Each packet typically comprises 1]header information associated with a particular protocol, and 2] payloadinformation that follows the header information and contains informationthat may be processed independently of that particular protocol. In someprotocols, the packet includes 3] trailer information following thepayload and indicating the end of the payload information. The headerincludes information used by the protocol. Often, the data in thepayload for the particular protocol includes a header and payload for adifferent protocol associated with a different layer of detail forinformation exchange. The header for a particular protocol typicallyindicates a type for the next protocol contained in its payload. Theprotocol in the payload is said to be encapsulated in the protocol ofthe header for the payload.

The headers included in a packet traversing multiple heterogeneousnetworks, such as the Internet, typically include a physical (layer 1)header, a data-link (layer 2) header, an internetwork (layer 3) headerand a transport (layer 4) header, as defined by the Open SystemsInterconnection (OSI) Reference Model. The OSI Reference Model isgenerally described in more detail in Section 1.1 of the reference bookentitled Interconnections Second Edition, by Radia Perlman, publishedSeptember 1999.

The internetwork header provides information defining the source anddestination address within the network. Notably, the path may spanmultiple physical links. The internetwork header may be formattedaccording to the Internet Protocol (IP), which specifies IP addresses ofboth a source and destination node at the end points of the logicalpath. Thus, the packet may “hop” from node to node along its logicalpath until it reaches the end node assigned to the destination IPaddress stored in the packet's internetwork header.

Routers and switches are network devices that determine whichcommunication link or links to employ to support the progress of datapackets through the network. A network node that determines which linksto employ based on information in the internetwork header (layer 3) iscalled a router.

Some protocols pass protocol-related information among two or morenetwork nodes in special control packets that are communicatedseparately and which include a payload of information used by theprotocol itself rather than a payload of data to be communicated foranother application. These control packets and the processes at networknodes that utilize the control packets are said to be in anotherdimension, a “control plane,” distinct from the “data plane” dimensionthat includes the data packets with payloads for other applications atthe end nodes.

A link-state protocol is an example of a routing protocol, which onlyexchanges control plane messages used for routing data packets sent in adifferent routed protocol (e.g., IP). As stated in the background, toreduce the consumption of network resources and improve scalability,some routing protocols divide a large network up into smallersubnetworks. For example, the Open System Interconnection (OSI) protocolsuite and the Open Shortest Path First (OSPF) routing protocol divide anetwork into domains and areas. A domain is a portion of a network underthe network administration of a single authority, such as an enterpriseor Internet service provider (ISP). A domain is also called anautonomous system (AS). A domain is divided into areas. Each area is agroup of contiguous subnetworks and attached end nodes specified by anetwork administrator, usually manually. In OSI, routers within an AScommunicate with each other using an intermediate system to intermediatesystem (IS-IS) protocol. According to IS-IS, routing within an area(level 1 routing) uses link-state data that distinguishes each link oneach router in the area. Routing between areas (level 2 routing) goesthrough a level 2 router that aggregates the addresses reachable throughthat level 2 router. By aggregating routing information for addressesreachable over many links of a level 2 router, the amount of networkresources consumed to maintain link-state data and make routingdecisions can be reduced and network scalability can be enhanced. Asstated in the background, the division of routers into areas isconventionally a manual process performed by human networkadministrators.

In an internetwork, networks in different autonomous systems (AS) alsoroute data packets among each other. In general, the network nodes in anautonomous system are manually configured with an Autonomous Systemidentifier (ASID). Routing information for an AS is summarized at itsboundaries with one or more other ASs at intermediate network nodescalled border gateway nodes or border gateway (BG) routers. Routinginformation shared within the borders of one AS is exchanged using aninterior gateway protocol (IGP). Example IGPs include the link stateprotocols OSPF and IS-IS described above. Another IGP, developed byCisco Systems of San Jose, Calif. for use in its routers, is theEnhanced Interior Gateway Routing Protocol (EIGRP).

A level 3 routing protocol is used to exchange route summary and routingpolicy information across AS borders. For example, the Border GatewayProtocol (BGP) is a level 3 routing protocol. The BGP sends summary andpolicy information between adjacent boundary gateway nodes in differentASs using the External BGP (EBGP). The BGP sends summary and policyinformation between different boundary gateways in the same AS using theInternal BGP (IBGP).

In general, efficient use of network resources depends on thecommunications links among nodes and the manually configured aggregationpoints. In the following, three sets of network conditions are comparedin terms of their suitability for effective aggregation.

FIG. 1A illustrates an example hierarchical network 101. Network 101include local area network (LAN) 110 a, LAN 110 b, LAN 110 c, LAN 110 d(collectively referenced herein as LAN 110), each connected to one ormore end nodes (not shown). Network 101 also includes multipleintermediate network nodes, such as routers, including node 121 a, node121 b, node 121 c, node 121 d, node 122 a, node 122 b, node 122 c, node122 d, node 122 e, node 122 f and node 122 g (collectively referencedherein as nodes 120). Nodes 120 include leaf nodes 121 a, node 121 b,node 121 c, node 121 d (collectively referenced herein as leaf nodes121) and transit nodes 122 a, node 122 b, node 122 c, node 122 d, node122 e, node 122 f and node 122 g (collectively referenced herein astransit nodes 122). Data plane traffic enters and leaves the network ofintermediate network nodes 120 at the leaf nodes 121, so the leaf nodes121 are said to terminate communications among the intermediate networknodes 120. Within the entire network 101, communications are terminatedat end nodes (not shown) on each LAN 110. By definition, any router thathas a communication link with only one other router is a leaf node;however, some leaf nodes (not shown) may have communication links withmultiple other routers. Leaf nodes are not used for passing traffic fromone router to another and are sometimes call stub routers. Intermediatenetwork nodes 120 that are not leaf nodes 121 are transit nodes 122.Transit nodes 122 route data traffic between leaf nodes.

Network 101 includes only direct communication link 130 a, link 130 b,link 130 c, link 130 d, link 130 e, link 130 f, link 130 g, link 130 h,link 130 i and link 130 j (collectively referenced herein as links 131).

A hierarchical network is one in which nodes are connected in a treestructure in which one root serves as a parent node for one or morechild nodes. Each child node has only one parent node, but may havezero, one or more child nodes of its own. A child node that has no otherchild node is a leaf node.

Network 101 is a hierarchical network because it can be represented by atree in which node 122 a is the root node with two child nodes—node 122b and node 122 c. Node 122 b is parent node to node 122 d and node 122e; node 122 c is parent node to node 122 f and node 122 g. Node 122 band node 122 c are in a second level of the hierarchy. Their childnodes, node 122 d, node 122 e, node 122 f, node 122 g are in the thirdlevel of the hierarchy. At the next level of the hierarchy are the leafnodes 121. Leaf node 121 a is the child of node 122 d, leaf node 121 bis the child node of node 122 e, leaf node 121 c is the child node ofnode 122 f, leaf node 121 d is the child node of node 122 g.

A property of a hierarchical network is that there is only one pathbetween any two end nodes. A path is a sequence of nodes connected bydirect communication links. Communication links that lead to multiplepaths between the same two end nodes are physically removed or ignoredin order to generate the hierarchical network. The Spanning Treeprotocol is a routing protocol that ignores communication links thatviolate the tree structure of a hierarchy.

An advantage of hierarchical networks is that loops are avoided. Anotheradvantage is that hierarchical networks lend themselves readily toaggregation. Data from nodes at lower levels of the hierarchy, closer tothe end nodes, can be combined or summarized and sent in combined formto nodes at higher levels, farther from the end nodes. For example, thenetwork addresses reachable through child node 122 d and child node 122e can be combined at parent node 122 b and presented to root node 122 awithout sharing information on how the addresses are distributed betweenthe child nodes of parent node 122 b. As a result, hierarchical networksscale up to large numbers of nodes very well.

A disadvantage of hierarchical networks is that they can lead tosub-optimal routing, e.g., taking many more hops to reach a destinationthan is necessary. For example, it is assumed for purposes ofillustration that node 122 e is close enough to node 122 f to form adirect connection with a communication link. If such a link wereavailable, data packets from LAN 110 b could be sent to LAN 110 c bytraversing only four nodes—node 121 b, node 122 e, node 122 f and node121 c. However, to maintain the hierarchy, such a link is eliminated aslong as link 130 f is in place; otherwise one of node 122 e and node 122f is the child of two parents. Thus traffic from LAN 110 b must traverseseven network nodes—node 121 b, node 122 e, node 122 b, node 122 a, node122 c, node 122 f and node 121 c. A purely hierarchical network is notresilient; when a communication link that provides the best path to adestination goes down, there are no other links to use to support a pathto the same destination. Quite often, network designs violate theprinciples of purely hierarchical design to provide some resilience.

FIG. 1B illustrates an example full mesh network 102. Network 102includes LAN 110 and nodes 120, as in network 101. However, network 102includes a full mesh of communication links among the nodes 122(collectively referenced herein as communication links 132), includingsix links between each transit node 122 and every other transit node122. For example, in addition to the communication links 131 depicted inFIG. 1A, network 102 communication links 132 include link 130 k, link130L, 130 m, link 130 n, among others.

A property of a full mesh network is that there are many paths betweenany two end nodes. An advantage of full mesh networks is that theyalways provide optimal routing, e.g., taking the fewest possible hops toreach a destination. For example, data packets from LAN 110 b could besent to LAN 110 c over the direct link 130 k between node 122 e and node122 f, which link is not available in hierarchical network 101. Fullmesh networks are also more resilient; when a communication link thatprovides the best path goes down, there are plenty of other links to useto support the next best path.

A disadvantage of full mesh networks is that loops are easy to form andmore complex logic must be included in routing protocols to avoid loops.Another disadvantage is that full mesh networks do not lend themselvesto aggregation. Many paths are available to circumvent any aggregationpoints. For example, it is assumed, as described above for thehierarchical network, that the network addresses reachable through node122 d and node 122 e are combined at node 122 b and presented to node122 a without sharing information on how the addresses are distributedbetween node 122 d and node 122 e. However, no path between any two leafnodes 121 goes through the aggregation point on node 122 b. Therefore nopaths benefit from the aggregation; and the aggregation becomessuperfluous. All nodes in network 102 will flood full topology data toeach other. As a result, full mesh networks do not scale up to largenumbers of nodes very well.

FIG. 1C illustrates an example preferred network 103. Network 103includes LAN 110 and nodes 120, as in network 101 and network 102.However, network 103 includes more communication links among the nodes122 (collectively referenced herein as communication links 133) than thehierarchical network 101 and fewer than the full mesh network 102. Inaddition to the communication links 131 depicted in FIG. 1A, network 103communication links 133 include link 130L, 130 m and link 130 n.Communication link 130 k (indicated by a broken line in FIG. 1C), amongothers, are not included. Network 103 combines the capacity to aggregateand scale well to large number of nodes offered by the hierarchicalnetwork 101 with the resiliency against link failure provided by thefull mesh network 102. The effectiveness of the aggregation depends onwhich interfaces are used to aggregate the network data (including dataplane data or control plane data or both). In the illustratedembodiment, aggregation point interface 151 is between node 122 b andlink 130 n; and aggregation point interface 152 is between node 122 cand link 130 n.

Although a particular number of LAN 110, router leaf nodes 121, routertransit nodes 122 and communication links 131, 132, 133 are depicted inFIG. 1A, FIG. 1B and FIG. 1C for purposes of illustration, in otherembodiments networks includes more or fewer communication links, LAN,and leaf nodes and transit nodes of the same or different intermediatenetwork node types.

Several different kinds of aggregation are possible at intermediatenetwork nodes. In the following, three types of example aggregation aredescribed.

FIG. 2A illustrates example data rate aggregation 201 through aninterface of a node. FIG. 2A depicts a node 210 (such as a transit node122), communication link 217 a, link 217 b and link 217 c (collectivelyreferenced as links 217) connected to interface 212 a, interface 212 band interface 212 c, respectively, on node 210. Link 217 a and link 217b are T1 communication links that support a maximum data rate of 1.544megabits per second (Mbps, a megabit=10⁶ binary digits called bits).Link 217 c, however is a T3 communication link that supports a muchlarger maximum data rate of 44.736 Mbps. Thus T3 link 217 c can handleall the data that can possibly come on T1 link 217 a and link 217 b andseveral other links (not shown). Data flowing out through interface 212c is a simultaneous combination of the data coming in through interfaces212 a and interface 212 b. That is, even if data comes in at near themaximum rate, 1.5 Mbps, on both link 217 a and link 217 b, all that datagoes out at a combined rate, 3.0 Mbps, through interface 212 c. Thistype of aggregation is called data rate aggregation or physicalaggregation. If link 217 c were a T1 link, no data rate aggregationoccurs on node 210, Data rate aggregation applies to both data planedata and control plane data.

Note that the aggregation is directional. Data flowing from link 217 aand link 217 b are aggregated onto link 217 c. Data coming in on link217 c is not aggregated with data on link 217 a or 217 b to go out theother T1 link. The direction of data aggregation 219 is indicated by anarrow in FIG. 2A.

Note that aggregation is associated with a particular interface on thenode and not with every interface on the node. Data coming in on link217 a and out on link 217 b is not aggregated. Only data going outthrough interface 212 c to link 217 c is aggregated. The interfacethrough which the aggregated data flows out is called the aggregationpoint. Thus, in FIG. 2A, interface 212 c is the aggregation point fordata rate aggregation on node 210.

FIG. 2B illustrates example topology aggregation 202 through aninterface of a node. FIG. 2B depicts a node 220 (such as a transit node122) and interface 222 a, interface 222 b and interface 222 c on node220 connected to separate communication links (not shown). Full topologydata, such as is flooded in an OSPF area, is received in message 227 aand message 227 b through interface 222 a and interface 222 b,respectively. Node 220, however, performs summarization, such as is doneon a level 2 router, and sends only reachability data to routers in adifferent area through interface 222 c in a reachability data message227 c. The reachability data in message 227 c indicates all the IPaddresses described in topology data received through interfaces 222 aand 222 b, but does not associate those addresses with either interface222 a or interface 222 b or an interface on any other node. Data flowingout through interface 222 c is a simultaneous combination of the datacoming in through interfaces 222 a and interface 222 b. This type ofaggregation is called topology aggregation. If outgoing message 227 ccarried full topology data, then no topology aggregation occurs on node220. Topology aggregation applies to control plane data.

As with data rate aggregation, topology aggregation is directional andassociated with an aggregation point at a particular interface and notwith every interface on the node. The direction of data aggregation 229is indicated by an arrow in FIG. 2B. Interface 222 c is the aggregationpoint interface on node 220. Full topology data received at interface222 a is output through interface 222 b

without aggregation. Similarly, full topology received at interface 222b is output through interface 222 a without aggregation,

In some embodiments, topology aggregation takes a different form.Instead of receiving full topology data message 227 a and message 227 bon interface 222 a and interface 222 b, respectively, one of thesecommunication links is absent and full topology data is received on onlyone interface, e.g., interface 222 a. The full topology data mayindicate other nodes upstream of the node connected directly tointerface 222 a and the interfaces on those nodes. Node 220, however,still performs summarization, such as is done on a level 2 router orlevel 3 router; and sends only address reachability data to routers in adifferent domain through interface 222 c in a reachability data message227 c. The reachability data in message 227 c indicates all the IPaddresses described in reachability data received through interfaces 222a, but does not associate those addresses with either interface 222 a oran interface on any other node upstream of interface 222 a.

In some embodiments, topology aggregation takes a still different form.Instead of receiving full topology data message 227 a and message 227 bon interface 222 a and interface 222 b, respectively, reachability dataonly is received on each interface. Node 220, however, performssummarization, such as is done on a level 3 router, and sends onlyreachability data to routers in a different domain through interface 222c in a reachability data message 227 c. The reachability data in message227 c indicates all the IP addresses described in reachability datareceived through interfaces 222 a and 222 b, but does not associatethose addresses with either interface 222 a or interface 222 b or aninterface on any other node.

FIG. 2C illustrates example address aggregation 203 through an interfaceof a node. FIG. 2C depicts a node 230 (such as a transit node 122) andinterface 232 a, interface 232 b and interface 232 c on node 230connected to separate communication links (not shown). Address data,such as is forwarded between OSPF areas in a reachability message, orwithin an area in a full topology message, is received in message 237 aand message 237 b through interface 232 a and interface 232 b,respectively. Node 230, however, performs address combination in whichmultiple hierarchical addresses or address ranges are combined into asingle address range. Node 230 sends a single IP address range torepresent multiple input addresses or ranges to routers in a differentarea or domain through interface 232 c in a combined address datamessage 227 c. The output message 237 c can also include one or moreother addresses or address ranges that represent other combined oruncombined addresses or ranges.

Certain types of addresses are well suited for combining in a singlerange. For example, IPv4 addresses are 32 bits. An octet is eight bitsthat can represent decimal values from 0 through 255. If contiguous IPv4addresses are assigned to end nodes on one or more neighboring LANs,those addresses will likely be reachable through the same interface atsome point downstream in the direction of data flow. Contiguous IPv4addresses can be combined in a single range.

IPv4 addresses are often expressed as four decimal values separated bydots, e.g., 10.1.0.0. A range of IP addresses are expressed by a basevalue followed by a slash and mask number. The mask number indicates thenumber of most significant bits that remain constant throughout therange. Thus a range of 255 contiguous IPv4 addresses is 10.1.0.0/24.During address aggregation, the address data indicting addresses10.1.0.0/24 and 10.1.1.0/24 are combined into the single range10.1.0.0/23. This type of aggregation is called address aggregation.

If outgoing message 237 c carried the same list of separate addresses oraddress ranges as received in address data 237 a and 237 b, then noaddress aggregation occurs on node 230. If two or more addresses oraddress ranges are combined into a single range in combined addressrange data message 237 c, then address aggregation is occurring. Addressaggregation applies to control plane data.

As with data rate aggregation and topology aggregation, addressaggregation is directional and associated with an aggregation point at aparticular interface and not with every interface on the node. Thedirection of data aggregation 239 is indicated by an arrow in FIG. 2C.Interface 232 c is the aggregation point interface on node 230. Addressdata received at interface 232 a is output through interface 232 bwithout aggregation. Similarly, address data received at interface 222 bis output through interface 222 a without aggregation,

In other embodiments, data aggregation is also performed at a node formultiple directions, and different interfaces are aggregation points fordifferent flow directions.

3.0 Method

According to various embodiments, a network of leaf nodes, transit nodesand communication links is evaluated or modified or both based on ameasure of aggregation.

FIG. 3 illustrates an example method 300 to evaluate and improve dataaggregation in a network. Although steps in FIG. 3 and any subsequentflow charts are shown in a particular order for purposes ofillustration, in other embodiments, one or more steps may be performedin a different order or overlapping in time, in series or in parallel,or one or more steps may be omitted or added, or changed in somecombination of ways. In some embodiments, method 300 is implemented on ageneral purpose computer, such as a network management system computer;in some embodiments method 300 is implemented on a router that receivestopology information for all other routers in a network to be evaluated.

In step 310, topology data is received. The topology data indicatescommunication links and nodes that interface with those links. Thetopology data also indicates the leaf nodes. A leaf node is either anintermediate network node that has a communication link with only oneother intermediate network node, or an intermediate network node that isso marked as a leaf node, such as stub routers that are not used forrouting data packets to other routers. For purposes of illustration, itis assumed that the topology data indicates the communication links 133and nodes 120 of network 103 depicted in FIG. 1C.

Any method may be used to receive this data. For example, in variousembodiments, the data is included as a default value in softwareinstructions, is received as manual input from a person on a local or aremote node, is retrieved from a local file or database, or is sent froma different node on a network, either in response to a query orunsolicited, or the data is received using some combination of thesemethods. In various embodiments, the person manually inputting the datais a network design trainee or a network certification instructor for ahypothetical network, or a network administrator or consultant for anactual network.

In step 312, aggregation data is received. The aggregation dataindicates aggregation interfaces that output aggregated data for aparticular type of aggregation. For purposes of illustration, it isassumed that the type of aggregation is topology aggregation performedby OSPF level 2 routers between areas. The first aggregation point 151is the interface between node 122 b and link 130 n for topology datathat describes an area A that includes node 122 b, node 122 d, node 122e, node 121 a and node 121 b. The second aggregation point 152 is theinterface between node 122 c and link 130 n for topology data thatdescribes area B that includes node 122 c, node 122 f, node 122 g, node121 c and node 121 d.

In step 320 a set of paths is determined that includes one path for eachpair of leaf nodes. Any method may be used to select the path. Forexample, an open shortest path first algorithm is used or a cost metricminimization method is used. For purposes of illustration, it is assumedthat all communications links involve the same cost and the lowest costpath is selected for each pair of leaf nodes. The resulting set oftwelve (12) paths determined during step 320 for the network 103 islisted in Table 1. The portion of the path that pass through anaggregation point in the direction of aggregation is underlined. Theaggregation point is listed in the third column.

TABLE 1 Set of paths for leaf nodes in network 102. Aggregation pointLeaf passed in aggregation node pair Path direction 121a > 121b121a-122d-122e-121b 121a < 121b 121b-122e-122d-121a 121a > 121c121a-122d-122b-122c-122f-121c First (151) 121a < 121c121c-122f-122c-122b-122d-121a Second (152) 121a > 121d121a-122d-122b-122c-122g-121d First (151) 121a < 121d121d-122g-122c-122b-122d-121a Second (152) 121b > 121c121b-122e-122b-122c-122f-121c First (151) 121b < 121c121c-122f-122c-122b-122e-121b Second (152) 121b > 121d121b-122e-122b-122c-122g-121d First (151) 121b < 121d121d-122g-122c-122b-122e-121b Second (152) 121c > 121d121c-122f-122g-121d 121c < 121d 121d-122g-122f-121c

In step 330, a measure of aggregation is determined based on a number ofaggregated paths that pass through an aggregation point within the setof paths. During step 330, aggregation paths are identified among theset of paths. This portion of step 330 may be done in parallel with step320 to determine the paths, in some embodiments. For example, the pathsbetween 121 a and 121 b do not pass through the aggregation points 151or 152, therefore the paths between 121 a and 121 b are not aggregatedpaths. The path from 121 a to 121 c passes through aggregation point 151in the aggregation direction (output), therefore the path from 121 a to121 c is an aggregated path. The return path from 121 c to 121 a passesthrough aggregation point 152 in the aggregation direction (output),therefore the path from 121 c to 121 a is an aggregated path. Note thatif only aggregation point 151 were employed, the return path would gothrough the point 151 as input which is not in the aggregationdirection, and path 121 c to 121 a would not have been an aggregatedpath.

The measure of aggregation is based on the number of aggregated paths.As can be seen from Table 1, there are eight (8) aggregated paths amongthe set of 12 paths. Because the costs and aggregation points aresymmetric in the illustrated example, the set of paths and aggregationpaths could be determined for one direction and the results doubled forthe return direction. In this case there are six (6) pairs of leafnodes, disregarding direction, of which four (4) are connected byaggregated paths.

In some embodiments, the measure of aggregation is the ratio of thenumber of aggregated paths to the total number of paths in the set ofpaths. In the illustrated example, this ratio is 8:12 which can beexpressed as a fraction ⅔, a decimal 0.66667, or a percentage 67%. Inthe following, the measure of aggregation is expressed as a percentage.

In some embodiments, some paths are more important than others. Forexample, some paths carry a larger share of the traffic. Thus, in someembodiments, the measure of aggregation depends on a weighted number ofaggregated paths. For example, each aggregated path is weighted by thefraction of the actual or expected traffic to be carried by that path.For example, the measure of aggregation (MA) is given by Equation 1.

MA=[Σ _(i) wi*Ai]/[Σ _(i) wi](i=1, NP)   (1)

where NP is the number of paths, Ai is one if the ith path is anaggregated path and zero if the ith path is not an aggregated path, andwi is the weight assigned to the ith path. In the previous embodiment,wi=1 for all i. In an illustrated example, wi is the fraction of totaltraffic that takes a path. By definition Σ_(i) wi=1. Equally distributedtraffic provides 4weights of 1/12 (0.0833) for each path which leads tothe same result of MA=67%. However, if it is assumed for purposes ofillustration that non-aggregated path from 121 a to 121 b takes half thetraffic (w=0.5), and the remaining 11 paths evenly distribute the rest(i.e., have weights of 0.04545), then

MA=8*0.04545=36%.

In contrast, if aggregated path 121 a to 121 c takes half the traffic,then

MA=0.5+7*0.04545=82%.

In other embodiments, other weighing factors are used.

In step 340, the network is evaluated based on the measure ofaggregation. Too little aggregation is considered undesirable. Forexample, it is assumed for purposes of illustration that link 130 k isre-introduced into network 103 with the first aggregation point 151 andsecond aggregation point 152. Then all paths between leaf nodes will usethe link 130 k and avoid the aggregation points. The measure ofaggregation, MA, becomes zero. None of the best paths between leaf nodesuses the aggregation point. This is a poor network design.

Too much aggregation is also undesirable. For example, it is assumed forpurposes of illustration that hierarchical network 101 has fouraggregation points: two on node 122 b at interfaces with link 130 d andlink 130 e; and two on node 122 c at interfaces with link 130 f and link130 g. This effectively divides network 101 into four areas; a firstarea with node 122 b, node 122 d and node 121 a; a second area with node122 b, node 122 e and node 121 b; a third area with node 122 c, 122 fand 121 c; and a fourth area with node 122 c, 122 g, 121 d. Then allpaths between leaf nodes go through an aggregation point in thedirection of aggregation. The measure of aggregation, MA, becomes 1(100%). An excessive number of areas have been defined. This is also apoor network design.

According to one embodiment, a good design grade is computed based onthe MA ratio during step 340, as listed in Table 2.

TABLE 2 Example design grade based on measure of aggregation. MA rangeNetwork design grade  0% to 30% Poor 30% to 50% Good 50% to 75% VeryGood 75% to 85% Good  85% to 100% Poor

In some embodiments, during step 340, this network design grade isconferred on an engineer attempting to be certified for network designbased on a hypothetical network offered as provided as part of a reportanalyzing an actual network. In some embodiments, step 340 is omitted.

In the illustrated embodiments, method 300 includes step 350 to modifythe network based on the measure of aggregation. For example, the link130 k is removed from network 103 to increase the measure of aggregationfrom 0 to 67%. In some embodiments, step 350 includes making severalchanges to the network design and selecting the one modification thatgive a measure of aggregation closest to a target measure of aggregation(e.g., 65%). Then an actual network is changed to conform to the designthat gives the measure of aggregation closest to the target measure ofaggregation. In some embodiments, step 350 is omitted.

4.0 Implementation Mechanisms—Hardware Overview

FIG. 4 illustrates a computer system 400 upon which an embodiment of theinvention may be implemented.

Computer system 400 includes a communication mechanism such as a bus 410for passing information between other internal and external componentsof the computer system 400. Information is represented as physicalsignals of a measurable phenomenon, typically electric voltages, butincluding, in other embodiments, such phenomena as magnetic,electromagnetic, pressure, chemical, molecular atomic and quantuminteractions. For example, north and south magnetic fields, or a zeroand non-zero electric voltage, represent two states (0, 1) of a binarydigit (bit). A sequence of binary digits constitutes digital data thatis used to represent a number or code for a character. A bus 410includes many parallel conductors of information so that information istransferred quickly among devices coupled to the bus 410. One or moreprocessors 402 for processing information are coupled with the bus 410.A processor 402 performs a set of operations on information. The set ofoperations include bringing information in from the bus 410 and placinginformation on the bus 410. The set of operations also typically includecomparing two or more units of information, shifting positions of unitsof information, and combining two or more units of information, such asby addition or multiplication. A sequence of operations to be executedby the processor 402 constitutes computer instructions.

Computer system 400 also includes a memory 404 coupled to bus 410. Thememory 404, such as a random access memory (RAM) or other dynamicstorage device, stores information including computer instructions.Dynamic memory allows information stored therein to be changed by thecomputer system 400. RAM allows a unit of information stored at alocation called a memory address to be stored and retrievedindependently of information at neighboring addresses. The memory 404 isalso used by the processor 402 to store temporary values duringexecution of computer instructions. The computer system 400 alsoincludes a read only memory (ROM) 406 or other static storage devicecoupled to the bus 410 for storing static information, includinginstructions, that is not changed by the computer system 400. Alsocoupled to bus 410 is a non-volatile (persistent) storage device 408,such as a magnetic disk or optical disk, for storing information,including instructions, that persists even when the computer system 400is turned off or otherwise loses power.

Information, including instructions, is provided to the bus 410 for useby the processor from an external input device 412, such as a keyboardcontaining alphanumeric keys operated by a human user, or a sensor. Asensor detects conditions in its vicinity and transforms thosedetections into signals compatible with the signals used to representinformation in computer system 400. Other external devices coupled tobus 410, used primarily for interacting with humans, include a displaydevice 414, such as a cathode ray tube (CRT) or a liquid crystal display(LCD), for presenting images, and a pointing device 416, such as a mouseor a trackball or cursor direction keys, for controlling a position of asmall cursor image presented on the display 414 and issuing commandsassociated with graphical elements presented on the display 414.

In the illustrated embodiment, special purpose hardware, such as anapplication specific integrated circuit (IC) 420, is coupled to bus 410.The special purpose hardware is configured to perform operations notperformed by processor 402 quickly enough for special purposes. Examplesof application specific ICs include graphics accelerator cards forgenerating images for display 414, cryptographic boards for encryptingand decrypting messages sent over a network, speech recognition, andinterfaces to special external devices, such as robotic arms and medicalscanning equipment that repeatedly perform some complex sequence ofoperations that are more efficiently implemented in hardware. Logicencoded in one or more tangible media includes one or both of computerinstructions and special purpose hardware

Computer system 400 also includes one or more instances of acommunications interface 470 coupled to bus 410. Communication interface470 provides a two-way communication coupling to a variety of externaldevices that operate with their own processors, such as printers,scanners and external disks. In general the coupling is with a networklink 478 that is connected to a local network 480 to which a variety ofexternal devices with their own processors are connected. For example,communication interface 470 may be a parallel port or a serial port or auniversal serial bus (USB) port on a personal computer. In someembodiments, communications interface 470 is an integrated servicesdigital network (ISDN) card or a digital subscriber line (DSL) card or atelephone modem that provides an information communication connection toa corresponding type of telephone line. In some embodiments, acommunication interface 470 is a cable modem that converts signals onbus 410 into signals for a communication connection over a coaxial cableor into optical signals for a communication connection over a fiberoptic cable. As another example, communications interface 470 may be alocal area network (LAN) card to provide a data communication connectionto a compatible LAN, such as Ethernet. Wireless links may also beimplemented. For wireless links, the communications interface 470 sendsand receives electrical, acoustic or electromagnetic signals, includinginfrared and optical signals, that carry information streams, such asdigital data. Such signals are examples of carrier waves.

The term computer-readable medium is used herein to refer to any mediumthat participates in providing information to processor 402, includinginstructions for execution. Such a medium may take many forms,including, but not limited to, non-volatile media, volatile media andtransmission media. Non-volatile media include, for example, optical ormagnetic disks, such as storage device 408. Volatile media include, forexample, dynamic memory 404. Transmission media include, for example,coaxial cables, copper wire, fiber optic cables, and waves that travelthrough space without wires or cables, such as acoustic waves andelectromagnetic waves, including radio, optical and infrared waves.Signals that are transmitted over transmission media are herein calledcarrier waves.

Common forms of computer-readable media include, for example, a floppydisk, a flexible disk, a hard disk, a magnetic tape, or any othermagnetic medium, a compact disk ROM (CD-ROM), a digital video disk (DVD)or any other optical medium, punch cards, paper tape, or any otherphysical medium with patterns of holes, a RAM, a programmable ROM(PROM), an erasable PROM (EPROM), a FLASH-EPROM, or any other memorychip or cartridge, a carrier wave, or any other medium from which acomputer can read.

Network link 478 typically provides information communication throughone or more networks to other devices that use or process theinformation. For example, network link 478 may provide a connectionthrough local network 480 to a host computer 482 or to equipment 484operated by an Internet Service Provider (ISP). ISP equipment 484 inturn provides data communication services through the public, world-widepacket-switching communication network of networks now commonly referredto as the Internet 490. A computer called a server 492 connected to theInternet provides a service in response to information received over theInternet. For example, server 492 provides information representingvideo data for presentation at display 414.

The invention is related to the use of computer system 400 forimplementing the techniques described herein. According to oneembodiment of the invention, those techniques are performed by computersystem 400 in response to processor 402 executing one or more sequencesof one or more instructions contained in memory 404. Such instructions,also called software and program code, may be read into memory 404 fromanother computer-readable medium such as storage device 408. Executionof the sequences of instructions contained in memory 404 causesprocessor 402 to perform the method steps described herein. Inalternative embodiments, hardware, such as application specificintegrated circuit 420, may be used in place of or in combination withsoftware to implement the invention. Thus, embodiments of the inventionare not limited to any specific combination of hardware and software.

The signals transmitted over network link 478 and other networks throughcommunications interface 470, which carry information to and fromcomputer system 400, are example forms of carrier waves. Computer system400 can send and receive information, including program code, throughthe networks 480, 490 among others, through network link 478 andcommunications interface 470. In an example using the Internet 490, aserver 492 transmits program code for a particular application,requested by a message sent from computer 400, through Internet 490, ISPequipment 484, local network 480 and communications interface 470. Thereceived code may be executed by processor 402 as it is received, or maybe stored in storage device 408 or other non-volatile storage for laterexecution, or both. In this manner, computer system 400 may obtainapplication program code in the form of a carrier wave.

Various forms of computer readable media may be involved in carrying oneor more sequence of instructions or data or both to processor 402 forexecution. For example, instructions and data may initially be carriedon a magnetic disk of a remote computer such as host 482. The remotecomputer loads the instructions and data into its dynamic memory andsends the instructions and data over a telephone line using a modem. Amodem local to the computer system 400 receives the instructions anddata on a telephone line and uses an infra-red transmitter to convertthe instructions and data to an infra-red signal, a carrier wave servingas the network link 478. An infrared detector serving as communicationsinterface 470 receives the instructions and data carried in the infraredsignal and places information representing the instructions and dataonto bus 410. Bus 410 carries the information to memory 404 from whichprocessor 402 retrieves and executes the instructions using some of thedata sent with the instructions. The instructions and data received inmemory 404 may optionally be stored on storage device 408, either beforeor after execution by the processor 402.

In some embodiments, computer system 400 is configured as a router; andeach of one or more ASIC 420 is configured as a switching system withone or more communications interfaces, like communications interface470, and network links, like network link 478, to one or more localnetworks, like local network 480. In a computer system configured as arouter, it is common to omit one or more human interface devices, suchas pointing device 416, input device 412, and display 414.

5.0 Extensions and Alternatives

In the foregoing specification, the invention has been described withreference to specific embodiments thereof It will, however, be evidentthat various modifications and changes may be made thereto withoutdeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A method comprising: receiving topology data that indicates aplurality of communication links and a plurality of intermediate networknodes in communication based on the plurality of communication links,wherein the plurality of intermediate network nodes includes a pluralityof leaf nodes that terminate communications and a plurality of transitnodes that facilitate the passage of information between leaf nodes;receiving aggregation point data that indicates all aggregation pointson the plurality of intermediate network nodes, wherein an aggregationpoint is an interface between a network node and a communication link,which interface outputs data that is a combination of data receivedthrough a plurality of different interfaces; determining a set of paths,wherein each path in the set connects a different pair of leaf nodes;and determining a measure of aggregation based on a number of aggregatedpaths of the set of paths; wherein an aggregated path passes through anaggregation point.
 2. A method as recited in claim 1, further comprisingevaluating, based on the measure of aggregation, a network that consistsof the plurality of communication links and the plurality ofintermediate network nodes and all the aggregation points.
 3. A methodas recited in claim 1, said step of determining the measure ofaggregation further comprising determining a ratio of the number ofaggregated paths to a number of paths in the set of paths.
 4. A methodas recited in claim 3, further comprising the step of determining that anetwork consisting of the plurality of communication links and theplurality of intermediate network nodes and all the aggregation pointsis less able to find shortest routes as the ratio increases toward avalue of one.
 5. A method as recited in claim 3, further comprising thestep of determining that a network consisting of the plurality ofcommunication links and the plurality of intermediate network nodes andall the aggregation points is less able to scale to large numbers ofintermediate network nodes as the ratio decreases toward a value ofzero.
 6. A method as recited in claim 3, further comprising the step ofdetermining that a network consisting of the plurality of communicationlinks and the plurality of intermediate 4network nodes and all theaggregation points is well designed when the ratio has a value in arange from about 0.5 to about 0.7.
 7. A method as recited in claim 1,further comprising the step of modifying a network consisting of theplurality of communication links and the plurality of intermediatenetwork nodes and all the aggregation points based on the measure ofaggregation.
 8. A method as recited in claim 1, wherein an aggregationpoint is an interface that outputs data at a maximum data rate that isgreater than a maximum data rate on each of a plurality of differentinterfaces on the same node.
 9. A method as recited in claim 1, whereinan aggregation point is an interface that outputs network addressreachability data that is a combination of network address reachabilitydata received through a plurality of different interfaces on one or morenodes and does not output data that indicates the plurality of differentinterfaces or the one or more nodes.
 10. A method as recited in claim 1,wherein an aggregation point is an interface that outputs networkaddress reachability data that is a combination of network addressreachability data received through a plurality of different interfacesand does not output data that indicates the plurality of differentinterfaces.
 11. A method as recited in claim 1, wherein an aggregationpoint is an interface that outputs a single network address range thatis a combination of multiple network addresses received through aplurality of different interfaces.
 12. A method as recited in claim 1,said step of determining the measure of aggregation further comprisingweighting each path that connects two different leaf nodes by a portionof data traffic to be carried by that path
 13. A method as recited inclaim 1, said step of determining a measure of aggregation furthercomprising determining a measure of aggregation wherein an aggregatedpath passes through an aggregation point in a direction of aggregation.14. An apparatus comprising: means for receiving topology data thatindicates a plurality of communication links and a plurality ofintermediate network nodes in communication based on the plurality ofcommunication links, wherein the plurality of intermediate network nodesincludes a plurality of leaf nodes that terminate communications and aplurality of transit nodes that facilitate the passage of informationbetween leaf nodes; means for receiving aggregation point data thatindicates all aggregation points on the plurality of intermediatenetwork nodes, wherein an aggregation point is an interface between anetwork node and a communication link, which interface outputs data thatis a combination of data received through a plurality of differentinterfaces; means for determining a set of paths, wherein each path inthe set connects a different pair of leaf nodes; and means fordetermining a measure of aggregation based on a number of aggregatedpaths of the set of paths; wherein an aggregated path passes through anaggregation point.
 15. Software encoded in one or more computer-readablemedia and, when executed, operable for: receiving topology data thatindicates a plurality of communication links and a plurality ofintermediate network nodes in communication based on the plurality ofcommunication links, wherein the plurality of intermediate network nodesincludes a plurality of leaf nodes that terminate communications and aplurality of transit nodes that facilitate the passage of informationbetween leaf nodes; receiving aggregation point data that indicates allaggregation points on the plurality of intermediate network nodes,wherein an aggregation point is an interface between a network node anda communication link, which interface outputs data that is a combinationof data received through a plurality of different interfaces;determining a set of paths, wherein each path in the set connects adifferent pair of leaf nodes; and determining a measure of aggregationbased on a number of aggregated paths of the set of paths; wherein anaggregated path passes through an aggregation point.
 16. Software asrecited in claim 15, wherein, when executed, is further operable forevaluating, based on the measure of aggregation, a network that consistsof the plurality of communication links and the plurality ofintermediate network nodes and all the aggregation points.
 17. Softwareas recited in claim 15, said step of determining the measure ofaggregation further comprising determining a ratio of the number ofaggregated paths to a number of paths in the set of paths.
 18. Softwareas recited in claim 17, wherein, when executed, is further operable fordetermining that a network consisting of the plurality of communicationlinks and the plurality of intermediate network nodes and all theaggregation points is less able to find shortest routes as the ratioincreases toward a value of one.
 19. Software as recited in claim 17,wherein, when executed, is further operable for determining that anetwork consisting of the plurality of communication links and theplurality of intermediate network nodes and all the aggregation pointsis less able to scale to large numbers of intermediate network nodes asthe ratio decreases toward a value of zero.
 20. Software as recited inclaim 17, wherein, when executed, is further operable for determiningthat a network consisting of the plurality of communication links andthe plurality of intermediate network nodes and all the aggregationpoints is well designed when the ratio has a value in a range from about0.19 to about 0.21.
 21. Software as recited in claim 15, wherein, whenexecuted, is further operable for modifying a network consisting of theplurality of communication links and the plurality of intermediatenetwork nodes and all the aggregation points based on the measure ofaggregation.
 22. Software as recited in claim 15, wherein an aggregationpoint is an interface that outputs data at a maximum data rate that isgreater than a maximum data rate on each of a plurality of differentinterfaces on the same node.
 23. Software as recited in claim 15,wherein an aggregation point is an interface that outputs networkaddress reachability data that is a combination of network addressreachability data received through a plurality of different interfaceson one or more nodes and does not output data that indicates theplurality of different interfaces or the one or more nodes.
 24. Softwareas recited in claim 15, wherein an aggregation point is an interfacethat outputs network address reachability data that is a combination ofnetwork address reachability data received through a plurality ofdifferent interfaces and does not output data that indicates theplurality of different interfaces.
 25. Software as recited in claim 15,wherein an aggregation point is an interface that outputs a singlenetwork address range that is a combination of multiple networkaddresses received through a plurality of different interfaces. 26.Software as recited in claim 15, said step of determining the measure ofaggregation further comprising weighting each path that connects twodifferent leaf nodes by a portion of data traffic to be carried by thatpath
 27. Software as recited in claim 15, said step of determining ameasure of aggregation further comprising determining a measure ofaggregation wherein an aggregated path passes through an aggregationpoint in a direction of aggregation.